In my opinion being able to defend yourself is less about which art you study and more about how developed your martial mind is. Some people are born who are just naturally martial minded, some become so though training, no doubt some people never do.
That said different arts have different skill sets which reflect the arts stratagy and mindset and having a broad skill set and set of stratagies is always good. You should always have one art as a your basis though and in my opinion Aikido is excellent for that.